1. Field of the Invention
The present invention relates to printheads of the type employing a row of uniformly spaced light emitting diodes (LEDS) that can be individually energized to expose a photoreceptor or other information receiving medium and, more particularly, to means for supporting the light emitting diode array chips and other electronic components incorporated in such a printhead.
2. Description of the Prior Art
A printhead of the type to which the present invention is directed comprises a row of uniformly spaced light emitting diodes (LEDS) that can be individually energized to expose a photoreceptor or other information receiving medium to reproduce an image pattern. A typical LED array of this type for standard DINA4 paper dimensions would be about 216 mm long. The individual light sources are very small and very closely spaced, e.g. 160 per cm, which makes it impossible at the present state of the art to provide a full length array in one piece. Accordingly, the array comprises a number of individual LED array chips, each being typically less than 10 millimeters long, which are mounted in endwise relation to one another to provide the entire array.
To control the energization of the individual LED sites, each LED chip is connected along each edge to one of the corresponding control chips and each control chip, in turn, is connected along its other edge to circuit board means which provide for electrical connections between the printhead and other electrical components of the machine in which it is employed.
In most previously known printheads of this type, the LED array chips, the driver chips and the circuit board means are all supported to a wide flat face of a support bar of generally rectangular cross section, which is provided on its opposite face with a finned metal heat sink or the like to dissipate heat from the bar. The bar itself is typically made of metal, e.g. stainless steel, having approximately the same coefficient of expansion as that of the support material of the LED array chips, which is usually galium arsenide. Because the light emitting sites of the LED chips must be very accurately co-planar so that the emitted light can be precisely focused by a so-called Selfoc lens, which has a very short focal length, the surface of the support bar supporting the LED array chips must be of correspondingly precise flatness and must maintain that flatness notwithstanding the heat transmitted to the bar from the chips during operation of the printhead.
The typical use of a wide support bar having the chips mounted along its wide face presents two significant problems in achieving and maintaining the required co-planar location of the light emitting sites. First, because the required thermal expansion characteristics of the bar, the stainless steel or other metal alloy that must be used is typically difficult to machine to the required tolerances. Secondly, because the chips, particularly the LED array chips, transmit considerable heat to the bar along the central region of its flat support face, the resulting thermal gradients through the bar tend to bow it both along its longitudinal axis and also traversely to that axis. Accordingly, the support surface is not simply bowed in one direction but tends to bulge, which makes it very difficult to predict and prevent such distortion by conventional cooling means.
U.S. Pat. No. 3,701,123, issued on Oct. 24, 1972 discloses an integrated circuit module comprising a row of individual LED chips mounted along the narrow edge of a support bar provided on one of its wide faces with a control chip and with wiring means connecting the control chip to the LED chips. A plurality of such bars are stacked together with their wide faces in confronting relation to provide a two-dimensional visual display. However, in the disclosed construction, no means are provided for dissipating heat from the support bar and, because the device is intended for use only as a visual display, there are no stringent requirements that the LED sites be co-planar.
U.S. Pat. No. 4,506,272 discloses a thermal printhead comprising a row of small heater elements located along a cylindrical bar which, in turn, is supported by a concave narrow edge of a support bar sandwiched between substrate members carrying control chips and other electrical elements for energizing the heater elements. No means are disclosed for dissipating heat from the thermal printhead and such a device inherently does not involve maintaining the heater elements in co-planar relationship to an extremely high degree of accuracy, because the paper being printed can be in contact with those elements and can conform slight distortions or inaccuracies.